Many devices have been proposed the purpose of which is to dose a flowing fluid with an additive. The additive may be added to water for purification purposes, or may be a fertilizer, herbicide or other substance added to irrigation water. It may also be a lubricant added to the air flowing to pneumatic apparatus.
Devices which make use of a collapsible bag on which the following fluid impinges are known from U.S. Pat. Nos. 3,724,601 and 4,212,373. In neither of these constructions is any provision made for enabling the rate of flow of additive to be adjusted, nor for compensating for variations in the rate of flow of the fluid, nor for preventing the bag filling with flowing fluid after the air or water supply has been switched off.
The present invention seeks to provide devices of the general type described above, which are of more sophisticated form and have more useful operating characteristics.